The importance of thin films in both science and technology has created a need to better understand films. An active area of basic corrosion research is the study of passive film formation and breakdown aimed at improving metallic resistance to attack (K. Sugimoto and S. Matsuda, "Analysis of Passive Films on Austeno-Ferritic Stainless Steel by Microscopic Ellipsometry", Journal of the Electrochemical Society, vol. 130, No. 12, December 1983 and K. Sugimoto, S. Matsuda, Y. Ogiwara, and K. Kitamura," Microscopic Ellipsometric Observation of the Change in Passive Film on 18Cr-8Ni Stainless Steel with the initiation of Growth and Pit", Journal of the Electrochemical Society, vol. 132, No. 8, August 1985). Processes such as passive film changes during breakdown is one such area where a better understanding is needed. One area in which a better understanding would be useful is in the field of corrosion resistance. Corrosion resistance on many metals results from a thin passive oxide film which forms on a surface and protects the metal substrate. When the passive film layer is destroyed by localized chemical species, such as chloride ions, or mechanical abrasion, corrosion attack can occur.
Ellipsometry has proven to be a standard tool for the measurement of thin film thickness which has been restricted to a limited number of measurements on a surface at low spatial resolution. Up until now it has not been possible to image rapid film changes such as those that occur in an aqueous electrolyte under electrochemical control.
When spatially resolved images of film thickness variations are needed, the beam is focused to a small spot size and scanned over the surface (J. Kruger, "Applications of Ellipsometry to Electrochemistry", Advances in Electrochemistry and Electrochemical Engineering, vol. 9, Delahay and Tobias, John Wiley and Sons, New York, 1973; C. L. McBee and J. Kruger, "Events Leading to the Initiation of the Pitting of Iron", Localized Corrosion NACE-3, 1974; and C. L. McBee and J. Kruger, "Ellipsometric-Spectroscopy of Films Formed on Metals in Solution", Surface Science, vol. 16, 1969). This procedure produces very accurate ellipsometric results with good spatial resolution but poor temporal resolution due to excessive data acquisition times. By the time the entire area of the specimen had been scanned, the system had undergone serious change. One system available commercially ("Auto Gain Ellipsometers", Bulletin EE, page 13, Gartner Scientific Corp.) specifies an average of four seconds per point. To record a moderately sized image, 100 pixels square, more than eleven hours would be required. Another system ("Optical Sol-gel Coatings: Ellipsometry of Film Formation", A. J. Hurd and C. J. Brinker, J. Phys. France, Pre-print) exists which derives useful spatial information by tracking several distinct nulls in a film whose thickness was undergoing change. Because not all of the regions of the film satisfy the null conditions, this system does not yield a complete ellipsometric analysis.
High spatial resolution, finer than 25 microns, along with greatly increased measurement speed is needed to advance the study of rapidly changing thin films.